Function switch_context_fxsave 

unsafe extern "C" fn switch_context_fxsave(
    _old_rsp_ptr: *mut u64,
    _new_rsp_ptr: *const u64,
    _old_fpu_ptr: *mut u8,
    _new_fpu_ptr: *const u8,
)

rdi=old_rsp, rsi=new_rsp, rdx=old_fpu, rcx=new_fpu